Digital Subscriber Lines, DSL, are a common technology for providing digital communication over existing twisted copper pair subscriber lines. The subscriber line extends between two DSL modems. A first DSL modem is typically located in the customer's premises, and the second modem may be located at the local exchange (known as the ‘central office’ in US terminology), a street cabinet, or distribution point (sometimes known as ‘drop point’). Typically, the local exchange, street cabinet or distribution point includes a DSL Access Multiplexer, DSLAM (a form of aggregation transceiver device) comprising several DSL modems (one for each subscriber line). The DSLAM (at the exchange, cabinet or distribution point) connects the first DSL modem at the customer's premises to the Core Network and a Network Management System.
Network Operators employ Dynamic Line Management (DLM) systems to manage the DSL connections. Dynamic Line Management systems select a line profile (also just known as a “profile”) which sets constraints on various parameters for a DSL connection (for example, by setting absolute values for some parameters, and maximum and minimum values for others). The Network Operator may have a large number of profiles, each having a different set of parameters. The Network Operator applies the line profile to the DSL connection via the DSLAM.
During a training phase of a DSL connection (i.e. the synchronization phase), the two DSL modems make several measurements on the line and negotiate appropriate values for the parameters (for example, the data rate for the connection, power spectral density, margin, forward error correction parameters and carrier mask). The DSL line then establishes the connection based on the limits set by the profile and these measurements (i.e. the connection uses any absolute values specified by the profile, or a value between the maximum and minimum limits set by the profile based on the line measurements).
Once the training phase is complete, the two modems communicate with each other using these parameters. This normal operational phase is known as “Showtime”. If the DSL connection was trained during a low noise period, the DSL connection may have a data rate close to the maximum rate set by the profile. However, if the noise levels on the line subsequently increase, the DSL connection may be subject to a high Bit Error Rate, BER. If the BER increases to an unsustainable level, the line may need to resynchronize (i.e. “retrain”) to apply a more appropriate set of parameters (e.g. a lower data rate). These retraining periods are frustrating for the user due to loss of service for several minutes.
Conventionally, this problem was addressed by the Network Operator by selecting a profile which set a maximum limit on the data rate which was well below a maximum sustainable rate on that line (i.e. one which would experience a satisfactory BER even during high noise periods). This technique was known as rate capping, but would result in the data rate on the line being significantly compromised. As an improvement, a technique known as Signal to Noise Ratio, SNR, margin was introduced.
The SNR represents the amount a signal power level exceeds the background noise. The number of bits per channel on the DSL connection (i.e. the “bitloading”) is determined according to the SNR for that channel. If the SNR for a majority of channels on the DSL connection is high, then each channel may carry a large number of bits and the DSL connection may support a high data rate. However, if the SNR for a majority of channels is low, then each channel may only carry a relatively low number of bits and the DSL connection may only support a relatively low data rate.
During the training phase, the DSLAM measures the SNR for the line and determines the bitloading for each channel (and the corresponding data rate) based on the measured SNR minus a fixed tone-independent SNR margin (typically 6 dB, acting as a buffer should the noise levels fluctuate). Thus, if the measured SNR during initialization is relatively high (i.e. as the noise levels are low), the DSLAM calculates a correspondingly high data rate based on the measured SNR minus the SNR margin. However, if the SNR on the line subsequently decreases by a greater amount than the applied SNR margin, then the connection cannot sustain the data rate and the line must retrain. The Network Operator may assign a greater SNR margin during the training phase to address this problem, but (as with the rate capping technique), the resulting data rate is compromised.
The concept of Virtual Noise was introduced as an improvement to the SNR margin technique. Virtual Noise is a tone-dependent noise specified by the Network Operator and sent to the DSLAMs. During the training phase, the DSLAM measures the noise for the line, and uses a reference noise (i.e. the greater of either the measured noise on the line or the Virtual Noise) for calculating the bitloading on each tone. An SNR margin may also be applied (although typically much smaller than 6 dB) to compensate for any noise fluctuations. The Network Operators employ various algorithms to estimate a Virtual Noise for the DSL connection, which may include estimating the crosstalk on the line and historical noise data.
The Virtual Noise method was an improvement over the rate capping and SNR margin techniques. However, the existing Virtual Noise estimation algorithms do not accurately reflect the noise levels which may be experienced on the line. Accordingly, if the reference noise for a line during the training phase is a Virtual Noise level that has been estimated too high or too low, the DSLAM may still select an inappropriate data rate for the line.
It is therefore desirable to alleviate some or all of the above problems.